1. Field of the Invention
The present invention relates to a surface-emitting laser emitting light in a direction vertical to a layer surface, and a method of producing the same. In recent years, more and more studies have been conducted on optical communication and optical data links suitable for high-speed transmission. A surface-emitting laser is a key device for optical transmission, and its commercialization has been actively pursued, particularly for its potential for a lower threshold value, reduced power consumption, and lower production cost.
2. Description of the Related Art
FIG. 1 is a sectional view of a conventional vertical-cavity surface-emitting laser 10 (first prior art). According to FIG. 1, the surface-emitting laser 10 has a layer structure of a lower negative electrode 11, an n-type substrate 12, an n-type lower semiconductor multilayer film reflecting mirror 13, a resonator 20 including lower and upper clad layers 14 and 16 and an active layer 15 sandwiched there between, a current confinement layer 17, a p-type upper semiconductor multilayer film reflecting mirror 18, and an upper positive electrode 19 that are layered in the order described.
This structure has the advantage of being formable by a single crystal growth. However, in this structure, the electrical resistance of the entire surface-emitting laser 10 is increased because of the large electrical resistance of the semiconductor multilayer film reflecting mirrors 13 and 18 through which electric current is applied. Each of the semiconductor multilayer film reflecting mirrors 13 and 18 is a multilayer film formed by layering semiconductor films of two types of different refractive indices. Because there is a large band gap between the two types of semiconductor films, a large number of spikes are formed on each hetero-interface of the band structure. The spikes become barriers to carriers, particularly, holes, so as to increase electrical resistance. When electrical resistance is increased, power consumption increases, thus making it difficult to lower power consumption.
FIG. 2 is a sectional view of a conventional surface-emitting laser 30 to which electric current is applied from its sides (second prior art). According to FIG. 2, an upper electrode 31 of the surface-emitting layer 30 is provided around an upper multilayer film reflecting mirror 32 so that the electric current is applied to the active layer 15 without flowing through the upper multilayer film reflecting mirror 32. Therefore, the electrical resistance of the entire surface-emitting laser 30 can avoid the influence of the electrical resistance of the upper multilayer film reflecting mirror 32. Accordingly, the surface-emitting laser 30 can be reduced in resistance compared with the surface-emitting laser 10 of the first prior art.
In the surface-emitting laser 30, the current confinement layer 17 is formed between the upper electrode 31 and the active layer 15 in order to reduce the region of the active layer 15 to which region the electric current is applied. The current confinement layer 17 is composed of a conductive part 17-1 formed around the central axis Z of the surface-emitting laser 30 and an insulating part 17-2 formed around the conductive part 17-1. The electric current applied from the upper positive electrode 31 is restricted by the insulating part 17-2 of the current confinement layer 17 so as to flow through the conductive part 17-1. Accordingly, the electric current can be concentrated on a small region of the active layer 15, thereby increasing the efficiency of laser oscillation and at the same time decreasing a threshold current.
A computer simulation was conducted on the current density distributions of electric currents applied to the active layer 15 of the surface-emitting laser 30. FIG. 3 is a diagram showing the parameters, that is, the composition, film thickness, and doping density of each layer for the computer simulation. FIG. 4 shows the results of the computer simulation.
FIG. 4 is a graph showing the current densities of the electric currents applied to the active layer 15. In FIG. 4, the horizontal axis represents the distance from the central axis Z of the surface-emitting laser 30 and the vertical axis represents the current density. Further, the applied electric currents were 1 mA and 2 mA in the computer simulation.
FIG. 4 shows that the current density increases in the (radial) direction away from the central axis Z of the surface-emitting laser 30 to be maximized particularly at positions substantially below a sidewall surface 17-3, that is, the boundary between the conductive part 17-1 and the insulating part 17-2 of the current confinement layer 17. Further, this phenomenon becomes more apparent in the case of the 2 mA current. This is because the electric current is applied to the active layer 15, being concentrated on the sidewall surface 17-3 and its periphery. In such a case, the surface-emitting laser 30 is likely to oscillate with a high-order mode having a greater threshold gain than a basic mode. Then, the threshold current increases or an excess current flows, thus causing the problem of damaging the surface-emitting laser 30 before its laser oscillation.
According to a well-known method, the doping density is reduced for the conductive part 17-1 of the current confinement layer 17 and part of a first clad layer 33 formed directly thereon in order to control the concentration of the electric current on the periphery of the conductive part 17-1. FIG. 5 is a diagram showing the parameters of each layer of a surface-emitting laser according to such a method (third prior art). In FIG. 5, the same parameters as those of FIG. 3 are not shown.
According to FIG. 5, in contrast to the doping density of 1×1018 cm−3 of the first clad layer 33 of p-InGaP, the doping density of the p-InAlAs conductive part 17-1 of the current confinement part 17 is reduced to 1×1017 cm−3, and further, a p-InGaP second clad layer 35 of a doping density of 1×1017 cm−3 and a thickness of 0.05 μm is formed directly on the current confinement layer 17. A computer simulation was conducted using the parameters, that is, the composition, film thickness, and doping density of each layer shown in FIG. 5. FIGS. 6 and 7 show the results of the computer simulation.
FIG. 6 is a graph showing the current density distributions of electric currents applied to the active layer 15. FIG. 6 show the results of the 1 mA and 2 mA applied currents together with the results of the computer simulation of the second prior art for comparison. FIG. 7 is a graph showing the voltage-current characteristics and the differential resistance-current characteristics of the surface-emitting laser of the third prior art. FIG. 7 also shows the results of the computer simulation of the second prior art.
According to FIG. 6, the current density remains substantially constant in the direction away from the central axis of the surface-emitting laser up to positions substantially just below the sidewall surface 17-3 of the current confinement layer 17. Outside those positions, the current density decreases without having a peak maximum value. Accordingly, the problem of the current concentration on a part substantially right below the sidewall surface 17-3 can be eliminated by this method.
However, according to FIG. 7, when the applied current is increased, the voltage increases at a faster rate in the third prior art than in the second prior art, in which the second clad layer 35 whose doping density is decreased by one order of magnitude is not provided. That is, the differential resistance of the surface-emitting laser increases to cause the problem of an increase in its power consumption.
Further, according to another method, the electric resistance of the surface-emitting laser can be decreased by increasing the doping density of the first clad layer 33 formed directly on the second clad layer 35. However, such a method may cause free carrier absorption.